The present invention pertains to a detection and identification method, and more particularly to detection and identification of a variety of materials by the use of Mossbauer isotopes. This invention is especially suited for the detection of hidden explosive materials, as well as for the identification of the source or type of explosive, even after detonation. The invention is also useful for the detection and/or authentication of currency, tax stamps, gambling chips, classified documents and the like.
Much of the research effort in the art of explosive detection and identification has been directed toward the development of nuclear techniques in which the explosive material has incorporated therein, during manufacture, a substance which either emits radiation spontaneously, or can be induced to emit radiation, and the emitted radiation is detected with a suitable detection device. Similar methods have been proposed for marking classified documents to prevent theft, and for determining the authenticity of currency and identification documents.
The radioactive elements or compounds normally used as taggants in these prior art methods are of rather limited utility. Radioactive taggants must be employed in very limited quantities in order to avoid potential health and safety hazards. This quantitative limitation on the taggant has several drawbacks. In the first place, a detection method involving the use of small quantities of radioactive taggant may be defeated altogether since the material carrying the taggant may be shielded from detection. Secondly, without the aid of specialized and expensive detection equipment, it is difficult to discriminate between radiation emanating from the material sought to be detected and other background radiation.
In order to overcome the problem of discriminating between radiation emitted from a radioactive taggant and background radiation, it has been proposed to use taggants which emit a plurality of substantially time-coincident and direction-correlated gamma rays, such as positron emitters Na.sup.22 and Al.sup.26. Since gamma radiation is very penetrating the tagged material cannot readily be shielded from detection. Moreover, by using such positron emitters as taggants, it is possible to use a small quantity of taggant and yet differentiate between the gamma rays emitted from the taggant and cosmic radiation or emissions from innocuous articles, such as luminescent clock dials, since the probability of time and direction coincidence from such background sources of radiation is very small. The principal drawback of this method is that it requires several detectors for operation. These detectors may include crystal, liquid or solid scintillator detectors, photomultipliers and attendant logic, coincidence and discriminator circuitry. The expense of such apparatus makes it uneconomic for many small businesses which might advantageously employ such a system, for example, for authenticating currency received from customers. In addition, there are a relatively limited number of radioactive elements available which are capable of emitting time coincident gamma rays, thus making impossible all but the simplest identification tagging of explosive materials with these elements. For this and other reasons, the prior art explosive detection methods employing such taggants are not adaptable to the post-detonation identification of explosive materials as to type or source.
Another prior art nuclear technique for the detection of explosives involves adding a high cross-section, non-radioactive neutron absorber, such as boron, to the explosive, and thereafter irradiating the explosive with a neutron source, whereupon the neutron absorber emits a gamma ray. A suitable detection device monitors either the emitted gamma rays or the depression in the neutron field caused by the presence of the explosive. A similar technique has been disclosed for checking the authenticity of identity documents, e.g., identification cards. Aside from requiring the use of relatively expensive detection apparatus, this method presents a very serious health and safety risk in that the biological tolerance level of living tissues for neutrons is very low. Further, since the number of available neutron absorbers is relatively few, this method cannot be used effectively for the post-detonation identification of explosives.
Although certain problems are encountered in practicing the above-described nuclear techniques for detecting concealed weapons, explosives, etc., those techniques have advantages over non-nuclear techniques such as metal detectors, x-ray detectors and the like, because the latter require human intervention by trained operators and are often triggered by objects which are actually harmless, causing unnecessary inconvenience for the owner of the article being examined. Moreover, as a practical matter, the chemical reaction of detonation precludes virtually all but nuclear taggants from being used when it is desired to effect both pre-explosion detection and post-explosion identification.
The above-described nuclear detection techniques have also been proposed for use in anti-counterfeiting methods. Nuclear detection has advantages over the various non-nuclear, anti-counterfeiting techniques of the prior art, such as the use of complex printing patterns, paper having characteristic water-marks, or fluorescent printing inks. These non-nuclear techniques can be duplicated by persons reasonably skilled in the art of printing and generally require only a minimal investment in equipment and materials for producing bogus currency. A truly effective anti-counterfeiting method is one that not only requires uncommon skill or training in order to circumvent, but also involves the use of extremely sophisticated and expensive equipment for marking the currency.
The detection/identification method of the present invention overcomes the aforementioned shortcomings of the prior art nuclear techniques for the detection of concealed weapons, explosives, classified documents, etc., and satisfies the above criteria for an effective anti-counterfeiting method.